1. Field of the Invention
The present invention relates to a noise canceller and an AM receiving apparatus using the same.
2. Description of the Related Art
If a pulse noise P is superimposed on a receiving signal S received by a receiving antenna (see FIG. 8(a)), the pulse noise P is detected from the receiving signal S; the detected pulse noise P is peak-held; and when the peak-held value becomes greater than a predetermined threshold, a control signal H is generated which holds the level of the receiving signal S during a predetermined level hold period T (see FIG. 8(b)). A noise canceller has been proposed which holds the level of the receiving signal S during the level hold period T based on the control signal H (see FIG. 8(c)) to remove the pulse noise P from the receiving signal S (e.g., Japanese Patent Application Laid-Open Publication No. 6-112853).
With regard to the pulse noise P that may be superimposed on the receiving signal S, the pulse width fluctuates in accordance with the level of the receiving signal S received by the receiving antenna. For example, if the level is increased in the receiving signal S received by the receiving antenna, a relative level difference is reduced between the pulse noise P and the receiving signal S, which results in a waveform with the pulse noise P buried in the receiving signal S, and therefore, the pulse width of the detected pulse noise P is shortened in this case. On the other hand, if the level is reduced in the receiving signal S received by the receiving antenna, the relative level difference is enlarged between the pulse noise P and the receiving signal S, which results in a waveform with the pulse noise P appearing prominently in the receiving signal S, and therefore, the pulse width of the detected pulse noise P is elongated in this case.
Although the above phenomenon may occur, the level hold period T for each pulse noise P is fixed in a conventional noise canceller regardless of the magnitude of the level (field intensity) of the receiving signal S. Therefore, the level hold period T does not follow the fluctuations of the pulse width, and the level hold period T may become too long or too short for the pulse width of the pulse noise P.
If the level hold period T is too long for the pulse width of the pulse noise P, although the pulse noise P can certainly be removed, the distortion rate of the waveform of the receiving signal S accordingly worsens, causing deterioration of sound quality at the time of reproduction. On the other hand, if the level hold period T is too short for the pulse width of the pulse noise P, this is contrary to the original purpose of providing the noise canceller, which is to remove the pulse noise P.